Control device for a cooling circuit

ABSTRACT

The invention relates to a control device for a cooling circuit. The inventive control device ensures in a simple and reliable manner that the pressure tank of a reactor can be cooled even when a serious failure occurs, without having to resort to the use of external energy. According to the invention, at least one rod ( 9 ) is disposed below the bottom cap ( 6 ) of the reactor pressure tank ( 1 ). Said rod is connected to the emergency cooling system by means of a transmission system. The end of the rod ( 9 ) opposite the transmission system is arranged at a distance of approximately 3 cm from the bottom cap ( 6 ). A hydraulic system is used advantageously as a transmission system. The rod ( 9 ) is placed vertically on a hydraulic cylinder ( 7 ) arranged in a parallel position with respect to the axis of the reactor pressure tank ( 1 ), said cylinder being connected to the emergency cooling system. When several hydraulic cylinders ( 7 ) are used, they are connected to each other by means of hydraulic lines ( 8 ).

[0001] The invention relates to a control device for a cooling circuit. A preferred area of application of the invention is the emergency cooling of a reactor pressure vessel, especially for the vessel of a pressurized water reactor in the event of a core meltdown.

[0002] An arrangement for cooling a nuclear power plant, which automatically supplies the coolant depending on the pressure difference between the coolant and the reactor housing, is already known (DE 37 19 062 A1). A spurious release can be initiated with this technical solution.

[0003] It is also known that the temperature of the reactor pressure vessel may be measured and, when it exceeds a specified threshold, that the entry of water may be initiated by melting fusible plugs (A. Schaffrath, H. Ringel, Grundzüge passiver Sicherheitssysteme (Basic Principles of Passive Safety Systems) in: Neue Entwicklungen in der Kemergie zur Risikominimierung (New Developments in Minimizing the Risk of Nuclear Energy), Seminar VDI-GET & KTG, Dresden, May 11, 2000). This method of initiating the flow of coolant for controlling an incident is not suitable, at least not by itself.

[0004] It is an object of the invention to propose a control device for a cooling circuit, which is reliable and simple and ensures cooling of the reactor pressure vessel even in the event of a serious incident. At the same time, extraneous energy shall not be used.

[0005] Pursuant to the invention, the objective is accomplished with the distinguishing features given in the claims.

[0006] The flooding of the reactor pit is a known measure for dissipating large amounts of heat, which arise in the case of a nuclear core meltdown with the formation of a core melt lake in the bottom spherical cap of the reactor pressure vessel. It is a problem to ensure a passive initiation of the flooding of the outside of the reactor pressure vessel, which normally is dry, that is, surrounded by air or an inert gas. Passive means that externally controlled additional interventions in the plant or in mechanisms, based on extraneous energy, such as electrical circuits, sensors or motors, are not required.

[0007] On the basis of thermodynamics of a melt lake, the largest amount of heat enters the container wall in the upper region of the pool, which is named the focus region here. Because of the high temperatures in the focus region, the steel in this region has the least strength. At the same time, this wall region between the bottom spherical cap and the cylinder of the container must carry the whole weight of the bottom spherical cap and of the melt.

[0008] Significantly greater loads are caused by internal pressures, which may exist and can also leads to the highest stresses in the upper region of the bottom spherical cap and in the lower region of the cylinder. Because of these conditions, the vessel commences to deform in the regions of high load and low strength. On the other hand, regions lower down and further up and especially the center of the bottom spherical cap have lower temperatures and, accordingly, higher to very high strength.

[0009] In the focus region, when there is internal pressure, the container wall will bulge outward. In every case, that is, also without the additional load of internal pressure, the wall, when subjected to an appropriate thermal stress, which is equivalent to a small residual wall thickness, will be stretched in the downward direction. The whole bottom spherical cap is therefore shifted vertically in the direction of gravity.

[0010] In the regular operation of the power plant or in incidents of a different type, such as earthquakes, the inventive device does not have a negative effect, that is, it will not bring about any losses in efficiency in normal operation or additional sources of danger in any other state of the plant. Spurious releases are also excluded. In addition, the device is not an impediment during inspection work or repair work and can be realized relatively inexpensively. The simply constructed, reliable and passively acting control device can make do without outside energy.

[0011] The invention is described in greater detail below by means of an example of a hydraulic system. In the associated drawing,

[0012]FIG. 1 shows the lower part of a steel vessel, which can be used as a reactor pressure vessel, in the left part in the starting state and, in the right part, with a bottom part, lowered by high temperature and/or pressure, in section, and

[0013]FIG. 2 shows the plan view of the parts mounted below the steel vessel.

[0014] In the left half of FIG. 1, a starting state of a reactor pressure vessel 1 is shown in the case of a nuclear meltdown. The line 2 of rotational symmetry separates the left from the right part. In the right half of the Figure, the reactor pressure vessel 1 is shown after a deformation, for example, after an excessive temperature and/or pressure.

[0015] In the representation, the reactor pressure vessel 1 is filled approximately up to the transition between the bottom spherical cap 6 and the cylindrical part of the reactor pressure vessel 1 with a core meltdown lake 3. The region 4 characterizes the anticipated focus region in the wall of the reactor pressure vessel 1. Below the bottom spherical cap 6, there are four supporting disks 5, which are disposed in a star-shaped arrangement and do not contact the reactor pressure vessel 1 during a regular operation.

[0016] To ensure advantageous flow relationships, the existing devices in the ground region of the reactor pressure vessel 1 are included in the arrangement of the inventive control device. Accordingly, in each supporting disk 5, a recess is provided, under each of which a hydraulic cylinder 7 is disposed, which is connected over a hydraulic pipeline 8 with the other hydraulic cylinders 7. At the hydraulic piston of each hydraulic cylinder 7, a rod 9 is mounted, which protrudes through the recess of the supporting disk 5. The length of the rod 9 is such that its ends, averted from the hydraulic cylinder 7, are at the same distance of about 3 cm from the bottom spherical cap 6 of the reactor pressure vessel 1 in its control state. If the supporting disks 5, which are indicated here, are not present, the proposed control device can also be mounted at other components or constructed separately.

[0017] A hydraulic system, which operates according to the principle of communicating pipes, is a special embodiment. For example, if at several positions there is a hydraulic cylinder 7, the hydraulic piston of which, connected with a rod 9, is pushed in by the force of the vertically lowering bottom spherical cap, the desired passive system is available by way of the hydraulic pipelines 8. This system can be designed by defined piston paths, piston distances and piston cross-sectional areas so that the mechanisms, which are to be triggered, can be triggered only under the postulated conditions. For example, at least two pistons of the supporting arrangement must be pushed in, in order to extend one or more “initial pistons”, with which, for example, a dividing wall and/or a valve to the “In-Containment Refueling Water Storage Tank” (IRWST) is opened.

[0018] By initiating the flooding only after two pistons of the supporting arrangement are pushed in, that is, upon contact of the bottom spherical cap 6 with at least two supporting disks, unintentional flooding, for example, by contact with a supporting disk during repair work, can be avoided. 

1. A control device for a cooling circuit, especially for the emergency cooling of a reactor pressure vessel, the coolant being supplied to the cooling system depending on the onset of an event, wherein, below the bottom spherical cap (6) of the reactor pressure vessel (1), at least one rod (9) is disposed, which is connected over a transfer system with the emergency cooling device, the end of the rod (9), averted from the transfer system, being at a distance of about 3 cm from the bottom spherical cap (6).
 2. The control device of claim 1, wherein a hydraulic system is used as transfer system.
 3. The control device of claims 1 and 2, wherein the rod (9) is mounted perpendicularly at a hydraulic cylinder (7), which is disposed parallel to the axis of the reactor pressure vessel (1) and connected with the emergency cooling system.
 4. The control device of claim 3, wherein several hydraulic cylinders (7) are provided with rods (9), the hydraulic cylinders (7) being connected with one another by means of hydraulic pipelines (8).
 5. The control device of claim 4, wherein the hydraulic cylinders (7) are disposed in such a manner in the foundation (10) of the reactor pit that the rods (9) are passed through recesses of existing components. 